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Thread: Resistance, Voltage and Better Battery Life

  1. #1
    PV Master Team ECF (folding@home)
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    Default Resistance, Voltage and Better Battery Life

    Ive seen a lot of questions lately about resistance and how it affects battery life so Im reposting this article from another forum to hopefully clear up a couple of common misconceptions. This is the first time Ive ever done this so if its against the forum rules please let me know and I will delete it.

    This is primarily aimed at those wishing to build their own coils on a variable voltage device. However, it's also relevant to those using variable wattage, or even for those wondering what resistance of pre-made coils to buy for a variable device.

    Variable wattage is simply a way to avoid doing a bit of maths and having to adjust your voltage according to the resistance of your coils. It's a nice feature because it automates the process of getting the vape you want, but apart from that automation it doesn't offer any increase in performance over variable voltage, so let's ignore it for the purposes of this discussion and just talk about variable voltage.

    Low resistance is important if you have a mech mod, and thus we see a lot of people on the forums looking to make low resistance coils. If you want to vape at higher power on a mech mod, you can only do so by lowering your resistance.

    Let's assume that you're getting 4.2 volts from a new, fully charged battery. Working from Ohm's law, power is voltage squared divided by resistance. 4.2 squared is 17.64, so to get ten watts, you need a 1.764 ohm coil. If you want more than ten watts, you need to go lower, and as your battery performance deteriorates, you also need lower resistance. That's why if you have a mech mod and like a nice heavy vape, low resistance is the holy grail when building coils.

    Speaking personally, I fell into this trap when looking at resistance for a VV device, and I think a lot of people do. I've since learned a bit more, and am now seeing things in an entirely different light, and I'm going to explain why.

    If you have an Ego Twist, you can set your voltage up to 4.8 volts. To get ten watts from 4.8 volts, you need a 2.3 ohm coil. On a Vamo or a Provari, you can go up to 6 volts. That means you can get your ten watts from a 3.6 ohm coil.

    Why would you want to do this? Well let's do some maths.

    We talk about power, voltage and resistance quite a lot, but we don't see a lot of discussion of current. Current however, is what dictates your battery life.

    Current is voltage divided by resistance. In our examples above, the current on your Ego Twist would be 2.087 amps. On the Vamo or Provari, it's 1.667 amps. So if you are using higher voltage and resistance to get your power, you are using less current.

    Batteries are rated in milliamp hours. Let's say for instance you have an 800 mah battery. To get your battery life at a particular current, you divide the battery rating by the current to give you the time. In our example on the Ego Twist, you would get just under 23 minutes of holding the power switch down. On the Vamo or Provari however, you get just under 29 minutes. Hence we can see why lower current gives us longer battery life.

    What this means is that, once we know what power we want to vape at, the higher the resistance we can get away with, the longer our batteries are going to last. Whilst lower resistance is the holy grail of mech mod vaping, on a variable device, higher resistance is better, as long as it's not so high that you can't get the power you want.

    Sorry this is a long post, but I wanted to fully explore the reasoning behind my conclusion. I hope it helps others with a variable device to get an improved battery life.

    Caveat: there is a whole bunch of stuff I've left out, such as device accuracy, pulse width modulation, etc, etc, but the general principle of longer battery life from higher resistance coils at higher voltage holds true. Don't trust the figures and the readings you are getting from your equipment. Experiment until you get it right, but don't also assume that lower resistance is better, like I did.

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    I your post, great explanation and reasoning behind it. But I am wondering about something concerning the source voltage and amperage. A VV device needs to boost the voltage beyond the nominal cell voltage in the high resistance setup you've described. I think it may do this by drawing more current, so it may not actually be more efficient. Batteries don't really store amps, they store energy. Energy is proportional to power, so it has to come from somewhere. The squared term in watt's law is what makes it seem like higher voltage/resistance for a given wattage uses fewer amps, but we aren't working with an unlimited voltage source. The cell only puts out 4.2 by itself to start, gradually diminishing to 3.7 nominal then the device stops it at 3.2 or so, increasing the boost as the cell has less to give.
    Ohm's Law: V = I x Rtotal (I = current in amps)
    Watt's Law: W = V x I = V x V / Rtotal
    Equal parallel resistors (dual coils): Rtotal = Rsingle / 2

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    Quote Originally Posted by CloudZ View Post
    I your post, great explanation and reasoning behind it. But I am wondering about something concerning the source voltage and amperage. A VV device needs to boost the voltage beyond the nominal cell voltage in the high resistance setup you've described. I think it may do this by drawing more current, so it may not actually be more efficient. Batteries don't really store amps, they store energy. Energy is proportional to power, so it has to come from somewhere. The squared term in watt's law is what makes it seem like higher voltage/resistance for a given wattage uses fewer amps, but we aren't working with an unlimited voltage source. The cell only puts out 4.2 by itself to start, gradually diminishing to 3.7 nominal then the device stops it at 3.2 or so, increasing the boost as the cell has less to give.
    I think it depends on how efficient the boost circuit is. I know Provape claims the Provari is 95 percent efficient. Im sure theres a point where equilibrium and diminishing returns are achieved as well.

    I know on my Provari v2.5, I get a couple more hours battery life running 3ohm atties versus 2ohm.
    Last edited by zapped; 03-19-2014 at 02:10 PM.

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    Super Member Verified Member Dampmaskin's Avatar
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    I wrote a blog post about this topic the other day, and made a calculator where you can model how hard you're hitting the battery in your APV or mech, at any given resistance and selected voltage/wattage.
    Last edited by Dampmaskin; 03-19-2014 at 11:09 AM.
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    Quote Originally Posted by Dampmaskin View Post
    I wrote a blog post about this topic the other day, and made a calculator where you can model how hard you're hitting the battery in your APV or mech, at any given resistance and selected voltage/wattage.
    Awesome job! If you add a way to input MaH hours of our batteries and then divide that by the current it should show us how long a battery will last with a given combination of voltages and resistances.

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    Super Member Verified Member Dampmaskin's Avatar
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    Thanks.

    If you click the "Capacity" button, you can enter the mAh there, and get an estimated runtime and puff count.

    However, to get a realistic estimate of battery life in an APV, where power is the only constant value on the battery side, I need to know the Wh rating of the battery instead of the mAh rating.

    The calculator therefore converts mAh to Wh with Ohm's law using an "average voltage" of 3.6V. I don't know if this is the best "average voltage" to use, or if a different voltage would yield more accurate results. YMMV.
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    Thanks to all for this.

    My confusion... I bought a cheap mod, on/off safety button, 18350. I wanted something more than an ego 650. I just want to vape for as long as possible with a short size battery. What do I want in resistance? If I go low, my battery should not last as long, high and the vapor should drop as the battery volts drops but it should be slower to drop, correct?

    I also have an MVP, VV V3, and VV passthrough so I get the VV/VW stuff.
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    Super Member Verified Member Dampmaskin's Avatar
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    Yeah, you got it right.

    I'm not a veteran coil builder, but I've found my microcoils with cotton to be pretty energy efficient. My standard recipe is 30 AWG wrapped 10 times around a 1.3 mm safety pin. Touching loops. They usually end up at 2.0-2.2 ohms in my PT heads.

    With a loose cotton wick, they pop, sizzle, and produce good amounts of vapor in my experience, even at pretty low voltages. I was particularily impressed with one coil that had maybe 12 or 13 wraps and ended up at ~2.8 Ohm. It did an adequate job even at 3.2 V... that's only 3.66 Watts! Now I do like my wattage relatively low, but 3.66W? That's just ridiculous.

    It didn't last more than a day or so, before it started getting clogged though, requiring me to increase the voltage to more "normal" levels.
    Last edited by Dampmaskin; 03-19-2014 at 01:49 PM.
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